Did most of this work in:
https://github.com/alphagov/notifications-admin/pull/1118
> In pages specific to a service (e.g. dashboard and sub pages) the
> title needs to distinguish which service it applies to. This is mainly
> to give context to screen reader users who could be managing multiple
> services.
>
> Implementing this uses template inheritance:
>
> `page_title` includes `per_page_title` includes `service_page_title`
>
> ‘GOV.UK Notify’ is inserted into every page title.
>
> Pages that set `service_page_title` get the service name inserted too.
